Might be a worn first gear. First and reverse are the same if I remember right. It's VERY easy to drop the transmission on these and it's not overly heavy. You might hit up one of the parts guys in the marketplace for a new first gear then start pulling the tranny. I've got a full writeup, bolt by bolt, in my build thread
here.
Even if you don't need that gear, it's good to have extras on hand for later. If you do need it, it should be there by the time you're ready for it.